Most cryosurgical probes in use today are cooled by expanding a gas at high pressure in a cavity at the working tip of the probes whereby in accordance with the Joule-Thomson effect, the gas undergoes rapid cooling and the tip is brought quickly to its operating temperature.
Before using a cryosurgical probe of this type it has to be sterilised by a steam process. In this process probe moisture contamination may occur which will either prevent the probe from functioning at all or cause the tip to defrost after a short period of time by clogging the gas flow passages and the Joule Thomson nozzle through which the gas is caused to expand into the tip cavity.
It is normal practice therefore to purge the gas flow passages and Joule-Thomson nozzle of moisture contamination subsequent to the steam sterilisation process, by a purging method which involves passing a low pressure dry gas of the type eventually used for the cooling process, but at a higher pressure, through the probe gas flow passageways including the Joule-Thomson nozzle.
However for small diameter probes the required purging time is unacceptably high which reduces the efficiency of the device in the hands of the surgeon primarily because of the waiting time before it can be used.
Attempts have been made to reduce the purging time. For example, one method entails passing a low pressure gas through the probe in a flow direction reverse to the normal flow direction to effect cooling, i.e. the probe freeze cycle, and increasing the flow rate by drilling two small by-pass holes near the Joule-Thomson nozzle.
An alternative method disclosed in U.S. Pat. No. 3,613,689 utilises a valve which closes a by-pass hole provided close to the Joule-Thompson nozzle under the effect of high-pressure gas flow in the probe freeze cycle, but which is constrained to move away from the by-pass hole under the effect of low pressure gas during the probe purge cycle or warming mode, so increasing the flow rate and consequently the purging time. In this design the gas flow direction in both the purge and freeze cycles is the same.
The increased purging time provided by the prior art is, however, still not short enough fully to satisfy operational requirements. Moreover the introduction of small holes in the vicinity of the Joule-Thomson nozzle has the undesirable effect of decreasing the pressure drop across the nozzle so reducing its efficiency and the cooling effect.